PSI - Issue 10

E. Cheilakou et al. / Procedia Structural Integrity 10 (2018) 25–32 E. Cheilakou et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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structural needs, HBM linear pattern strain gauges with constantan grid lengths in the rage from 6mm to 10mm were selected for steel applications. In the case of inhomogeneous materials such as concrete, it is commonly essential to comply with the recommendation to use a grid length at least five times as long the largest aggregate grain size, cor responding to lengths up to 150mm. Furthermore, the selection of higher 350-Ohm against 120-Ohm gauge resistance was preferable, in that it reduces the heat generation rate by a factor of three.

3.4. Integrated system

A typical strain gauge connection to the system is shown in Fig.5a, where a four-channel SGC is used to drive 1/4 bridge, 1/2 bridge or full bridge configurations of strain gauges. Each configuration is considered as one channel and allocates one of the 32 available current input channels provided by the system. An image of the integrated system in laboratory environment is presented in Fig.5b, demonstrating the connection of a single-channel SGC to the mini SMS board, used to drive a 1/4 bridge configuration of linear strain gauge bonded on a test metal plate.

a)

b)

Fig. 5. (a) Typical strain gauges configurations with mini SMS. (b) View of the integrated system in laboratory.

4. Experimental testing

For the experimental tests, several SG bridge systems were set-up and installed on test steel plates. A thorough preparation of the plates surface prior to the sensors mounting, careful sensors bonding, and proper application of protection were performed using appropriate accessories and in accordance with manufacturers ’ recommended installation procedures (Hoffman (1996)). Indicative types of bridge systems are presented below (Hoffman (1989)):  Quarter Bridge Strain Gauge Configuration: 1 active-gauge 3-wire system This system is suitable for ordinary uniaxial strain measurement providing an x1 output signal. An image of this configuration in lab is shown in Fig.6, illustrating a linear strain gauge with 6mm grid length bonded on a test metal plate. An additional identical configuration was prepared, that was covered with protective agents and placed in the field. A thermocouple was also attached on the plate in the field for recording ambient temperature.  Half Bridge Strain Gauge Configuration: Orthogonal 2-active-gauge system This system is used for axial strain measurement yielding an output sign al multiplied by the factors of (1+ν), where ν: Poisson’s ratio . This configuration is suitable for eliminating the error due to thermal output and compensate for temperature effects. An image of this configuration is shown in Fig.7, where two linear pre-wired strain gauges are bonded on a test metal plate at a right angle and connected to adjacent sides of the Wheatstone bridge circuit. The above configuration was further covered with protection and placed in the field.  Half Bridge Strain Gauge Configuration: Active-dummy 2-gauge system The active-dummy method uses the 2-gauge system, where an active gauge is bonded to the test object and an ident ical dummy gage is mounted on an unstrained specimen made of the same material as the test object and is sub-